Polymer electrolyte fuel cells (hereinafter referred to as “PEFCs”) generate electric power and heat at the same time by the electrochemical reaction between a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas such as air. Each cell provided in a PEFC has an MEA (Membrane-Electrode-Assembly) composed of a polymer electrolyte membrane and a pair of gas diffusion electrodes (an anode and a cathode); gaskets; and plate-shaped electrically-conductive separators. PEFCs are generally formed by stacking a plurality of such cells, sandwiching the stack of cells at the ends with end plates, and fastening the end plates to the cells with a fastening device.
The principal surfaces of each separator are provided with manifold holes (a reaction gas supply manifold hole and a reaction gas discharge manifold hole) that define a manifold for supplying the fuel gas or the oxidizing gas or a manifold for discharging the fuel gas or the oxidizing gas (these gases are referred to as “reaction gases”), respectively. The principal surface of each separator in contact with either of the gas diffusion electrodes is provided with groove-shaped reaction gas channels which allow its associated reaction gas to flow therein and which are communicated with these manifold holes.
While flowing through their associated reaction gas channels, the reaction gases are supplied to the MEA and consumed by the electrochemical reaction occurring within the MEA. Therefore, hydrogen concentration and oxygen concentration decreases in the downstream portions of the reaction gas channels, due to the consumption of the reaction gases. This causes a problem that, in the downstream portions of the reaction gas channels where gas concentration is low, the amount of generated power decreases so that a power generation distribution corresponding to the gas concentration arises within in each cell surface.
As an attempt to solve this problem, there has been known a fuel cell according to which improved power generation efficiency is achieved by designing the shape of the gas channels so as to ensure uniform gas concentration in the cell surfaces (see e.g., Patent Document 1). FIG. 22 is a schematic view showing a schematic configuration of a principal surface of a separator provided in a fuel cell disclosed in Patent Document 1.
As illustrated in FIG. 22, the separator 200 provided in the fuel cell disclosed in Patent Document 1 has a plurality of fluid channels (reaction gas channels) 201 to 203 (three fluid channels in FIG. 22). Each fluid channel is composed of a substantially L-shaped upstream portion that is communicated, at its upstream end, with an inlet (reaction gas supply manifold hole) 211; a downstream portion that is communicated, at its downstream end, with an outlet (reaction gas discharge manifold hole) 212; and a midstream portion that connects the downstream end of the upstream portion to the upstream end of the downstream portion. The fluid channels are formed in a spiral shape when viewed as a whole. Accordingly, neither the upstream portions nor the downstream portions of the fluid channels are concentrated in a particular portion of the separator 200 so that uniform reaction gas concentration can be ensured in the electrode surface.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. 10-284094